The present invention relates to ferromagnetic thin film memories, and more particularly, to ferromagnetic thin film memories in which the memory cell structure is configured to reduce demagnetizing fields.
Digital memories are used extensively in digital systems of many kinds including computers and computer system components, and digital signal processing systems. Such memories can be advantageously based on the storage of digital bits as alternative states of magnetization in magnetic materials in each memory cell, typically thin film materials. These may be ferromagnetic thin film memories which may provide access to information stored therein by either inductive sensing to determine the magnetization state, or by magneto-resistive sensing for such determination. Such ferromagnetic thin film memories may be provided on the surface of a monolithic integrated circuit to provide for convenient electrical interconnections between the memory cell and the memory operating circuitry.
Ferromagnetic thin film memory cells are usually made very small and packed very closely together to achieve significant density of stored bits, particularly when provided on a surface in a monolithic integrated circuit. The magnetic environment can become quite complex with fields in any one memory cell affecting the film portions in neighboring memory cells. Also, small ferromagnetic film portions in a memory cell can lead to substantial demagnetizing fields which can cause instabilities in the magnetization state desired in such a cell.
These magnetic effects between neighbors in an array of closely packed ferromagnetic thin film memory cells can be ameliorated to a considerable extent by providing a memory cell based on an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic memory film is provided. Such an arrangement provides a significant "flux closure" to thereby confine the magnetic fields arising in a cell to affecting primarily just that cell. This is considerably enhanced by choosing the separating material and the ferromagnetic memory film to be sufficiently thin.
Often such a digital memory is constructed by having a number of memory cells in series connected at bit structure junctures to one another in an end-to-end fashion. A series of current straps, or word lines, are often provided in an orthogonal layout to a series of connected bit structures so that a strap crosses each of the bit structures between the bit structure junctures. In a magnetoresistive memory, such straps or word lines are used both in the entering of and the sensing of information in the bit structures. This can be done by using currents in the word lines for setting, or for determining the existing, magnetization of bit structures in the memory.
However, with respect to magnetic fields generated by word lines over a bit structure there is no "flux closure." This is because the word line is over the top of the bit structure and so there is no closed magnetic path for magnetic fields in the bit structure around that word line. The result is that very large demagnetizing fields can occur in a bit structure both for entering information and for sensing information in that bit structure. The magnetic field set up by current through the word line will attempt to magnetize each portion of the ferromagnetic memory film on the two major surfaces of the separating material in the same direction. The resulting distribution of "free poles" at the ends of the bit structure then give rise to such demagnetizing fields. Such fields can seriously disrupt the operation of the memory.